In Southern Europe the philosophy of the 17th. century was a tug of war between the vertical vision of belief in authorities and the horizontal optic of reasoning for oneself. The fundamental change was epistemological with the general debate about how we know. Some responded by invoking God's revelation or traditional authorities like Aristotle or Plato and others were based on human reasoning.
It was René Descartes that first began to redirect the interests of post-medieval philosophers from theology and Renaissance thinking towards reflections based on reason that escaped from ecclesiastical confines and Greek philosophical authorities.
In his 1637 Descartes' Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences (Original French: Discours de la Méthode Pour bien conduire sa raison, et chercher la vérité dans les sciences) is both philosophical and biographical. The author published the book in Holland in French instead of the traditional Latin in order to spread its philosophical thought to a wider audience (a similar idea to Luther's democratisation of the Bible through translation into the vernacular).
In his work Metaphysical Meditations (1641), written in Latin, he began to use doubt as a method, including doubt about scientific conclusions, the existence of reality and his own body. (He argued that reality may be a dream - an idea shared by earlier authors such as Calderón, Cervantes, and Shakespeare.)
Descartes thought the only thing he could trust was his own doubt. To doubt he thought that there must be something that doubts: himself. He concluded: 'I think therefore I exist.'
Descartes' goal in his Metaphysical Meditations was to argue for the existence of God. Once accepted, he concluded that the outside world must exist. He then studied the existence of the soul and concluded that the mind and body were separate entities but that they interacted in the pineal gland at the base of the brain. However, he was not convinced by this answer and spent the rest of his life searching for a better one.
Descartes revolutionised medieval thought by questioning it. However, he does not seem to have found a way to completely avoid the transcendent vision embraced by the religious content of that world.
The main question Descartes addresses in his Discourse is that of scepticism, a problem which dates back to ancient Greece. On searching for a solid basis for philosophical thought Descartes found in the sceptic tradition a substantial foundation. He starts his philosophical reasoning beginning with doubt thus beginning from a new perspective and avoiding biased prejudices. Traditional Aristotelian thought moves from first principles to deduct truths and Descartes undermines this method by doubting everything that is outside reason. The contemporary British empirical philosophers used sense perception and reason to attain knowledge but Descartes' method is based on doubt and the denial of sensory experience.
Part 1 deals with the sciences. Descartes believes that people have enough common sense to be able to differentiate between truth and fiction and he states that error comes about by not following the right thinking. Therefore they need a method. He also affirms that travel enlightens the mind and that education clouds it.
Part 2 is a description of a revelation he had in a “stove-heated room” on a trip to Germany. While thinking on a variety of subjects he had an epiphany: the work of an individual is superior to that of a committee because the single thinker follows one plan towards a particular goal. Science consists of many ideas from different people and diverse times and so is flawed. He then offers rules for methodical thinking: do not believe anything you can't prove for yourself; reduce problems to their simplest parts; order thoughts from simple to complex; to solve a problem create a chain of reasoning. Descartes' individualist idea that we should think for ourselves began an important move away from the classical method of relying on the Aristotelian tradition.
“The first was never to accept anything for true which I did not clearly know to be such; that is to say, carefully to avoid precipitancy and prejudice, and to comprise nothing more in my judgment than what was presented to my mind so clearly and distinctly as to exclude all ground of doubt.”
Part 3 offers a moral code: obey the regulations of the country and religion and do not accept extreme opinions; not to allow doubt to sway his decisions; change himself not the world; explore all professions and decide which is best. Descartes finally decides that the search for truth through reasoning is at the least very useful. He places great faith in his control of his own mind and thinks that to change reality you only need to change your mind. For example if you want something and can't have it just decide not to want it. He became a spectator of events rather than an actor which almost amounts to a denial of his own physical existence.
“My third maxim was to endeavour always to conquer myself rather than fortune, and change my desires rather than the order of the world, and in general, accustom myself to the persuasion that, except our own thoughts, there is nothing absolutely in our power; so that when we have done our best in things external to us, all wherein we fail of success is to be held, as regards us, absolutely impossible: and this single principle seemed to me sufficient to prevent me from desiring for the future anything which I could not obtain, and thus render me contented.”
Part 4 offers proofs of the existence of God and the soul. Reflecting on the senses he concludes that they are not reliable and so separates the body from the soul which relies on reason. Doubting himself leads to the conclusion that he is imperfect but he can conceive of perfection and so judges that perfection does exist and names it God. He also believes that goodness and clear thinking have their roots in the divinity. Through reflection he gradually gains awareness of his own thinking. This will become the cornerstone of his philosophy: "I think, therefore I am".
“Finally, if there be still persons who are not sufficiently persuaded of the existence of God and of the soul, by the reasons I have adduced, I am desirous that they should know that all the other propositions, of the truth of which they deem themselves perhaps more assured, as that we have a body, and that there exist stars and an earth, and such like, are less certain;”
Part 5 discusses the differences between humans and animals. The latter cannot speak or reason and Descartes takes this as evidence of the human "rational soul". He concludes that the body-soul connection means that the soul has a life outside the body and is thus immortal.
Part 6 mentions the author's disagreements with the Church about the physical sciences. He also apologises for writing in the vernacular instead of Latin and sets the search for truth above fame and fortune.
Descartes thought that information received through the senses was unreliable. Starting from this doubt on sensory perception he decided to prove to himself the most basic assumptions before accepting them. He argued, based on dreams, that they seemed real while asleep. He experienced the warmth of a fire while sleeping, though the fire was not lit. From this incident he concluded that feeling a fire does not allow him to tell whether he is asleep or awake, conscious or dreaming. He reasons then that the senses are not trustworthy. Another argument put forward is that of the deceiving God. In this Descartes suggests that there may be an Almighty Being wanting to mislead him and inputting information directly to his brain so that he lived in an illusion. (The modern Matrix syndrome found in literary traditions such as Shakespeare, Calderón and Cervantes.) For Descartes these are thought experiments, not to be taken literally but designed to demonstrate that the senses can be deceived. If we can't put our trust in our senses then we cannot rely on deductions from sensory perceptions to know the world. This proposition was opposed by the British empiricists who held that all knowledge comes through the senses whereas the Cartesians argued that true knowledge comes only through applying pure reason.
“Those in whom the faculty of reason is predominant, and who most skillfully dispose their thoughts with a view to render them clear and intelligible, are always the best able to persuade others of the truth of what they lay down, though they should speak only in the language of Lower Brittany, and be wholly ignorant of the rules of rhetoric; and those whose minds are stored with the most agreeable fancies, and who can give expression to them with the greatest embellishment and harmony, are still the best poets, though unacquainted with the art of poetry.”
Descartes composed his Rules for the Direction of the Mind to demonstrate that certainty in science could be achieved through reasoning. The method was to break down all problems to their simplest parts and express them as equations. The goal was to eliminate as far as possible sensory perception as untrustworthy and by abstracting problems use objective reasoning to resolve them. He created a graphic depiction of problems using algebra and geometry and so reduced the shapes of real objects to a blueprint so that he could avoid sensory perception and approach them through reason.
Descartes argues that the act of thinking is a proof of existence and summarises this in the phrase "I think therefore I exist". He insists that he cannot be sure of anything about his existence but he is certain that he has thoughts and can use reason: if he has thoughts then an "I" must exist to perform the thinking. He believed that not the physical body but thought and reason were the essence of humanity since only humans can reason.
“... thence concluded that I was a substance whose whole essence or nature consists only in thinking, and which, that it may exist, has need of no place, nor is dependent on any material thing; so that “I”, that is to say, the mind by which I am what I am, is wholly distinct from the body, and is even more easily known than the latter, and is such, that although the latter were not, it would still continue to be all that it is.”
The subtitle of the original edition of the Discourse outlines his goal of communicability:
"In which the Author… explains the most abstruse Topics he could choose, and does so in such a way that even persons who have never studied can understand them.”
This underlines both his aim of divulging knowledge and his belief in the general human capacity to understand through reason. It is also one explanation of why he wrote the Discourse in French rather than in Latin.
“Good sense is the most evenly shared thing in the world [...] It indicates rather that the capacity to judge correctly and to distinguish the true from the false, which is properly what one calls common sense or reason, is naturally equal in all men, and consequently that the diversity of our opinions does not spring from some of us being more able to reason than others, but only from our conducting our thoughts along different lines and not examining the same things. For it is not enough to have a good mind, rather the main thing is to apply it well.”
"I must not doubt either that it is necessary that reality be formally in the causes of my ideas, although the reality which I consider in these ideas be only objective, nor think it sufficient that this reality be found objectively in their causes."
The scholastics distinguished formal and objective reality and Descartes subverts that tradition. Objective reality for him meant the ideas we have about phenomena, thus changing Plato's meaning when he uses as exchangeable the Form and Idea to refer to the nature of reality. In Cartesian understanding objective reality is so because it is a product of cognition. However, formal reality refers to the existence of the world independent of the mind. Knowledge of formal reality is to understand what a given object is regardless of our thoughts or feelings about it. Knowledge of objective reality lies in the mind since these phenomena are products of thought and owe their existence to cognition. (This is not dissimilar to Locke's distinction between primary qualities (a big red bus) and secondary qualities (our perception of the bus).
“although we very clearly see the sun, we ought not therefore to determine that it is only of the size which our sense of sight presents; and we may very distinctly imagine the head of a lion joined to the body of a goat, without being therefore shut up to the conclusion that a chimaera exists; for it is not a dictate of reason that what we thus see or imagine is in reality existent; but it plainly tells us that all our ideas or notions contain in them some truth.”